The present invention relates to a photosensitive resin composition, and more particularly, it relates to a photosensitive resin composition suitable for use as the top layer resist used in the two-layer resist process which requires a good resistance to oxygen plasma. The composition of the present invention will be used to form fine patterns necessary for the production of semiconductor elements.
The production of electronics components such as semiconductor elements, magnetic bubble memory, and integrated circuits involves the process of pattern formation. Heretofore, the pattern formation has been generally accomplished by using a photoresist which is sensitive to ultraviolet light or visible light. There are two types of photoresists: A negative type characterized by that the exposed part becomes insoluble in the developing solution; and a positive type characterized by that the exposed part becomes soluble in the developing solution. The negative type is superior to the positive type in sensitivity, adhesion to the substrate necessary for wet etching, and chemical resistance. For this reason, the photoresist of negative type has been predominant. However, with the advent of semiconductor elements having a high degree of integration and a pattern composed of extremely fine lines arranged close to each other, and also with the acceptance of dry etching for the patterns, there has arisen a demand for a photoresist having a high resolution and high resistance to dry etching. Nowadays, therefore, the photoresist of positive type has become to account for a large share. The most widely used photoresist of positive type is that of alkali development type which is based on an alkali-soluble novolak resin. It is outstanding in sensitivity, resolution, and dry etching resistance among many photoresists of positive type. It is explained in Kodak Microelectronics Seminor Proceedings, p. 116, (1976), by J. C. Strieter.
The photoresist of positive type, however, does not meet the recent requirements for higher density and higher integration and multilayer wiring structure of semiconductor elements, so long as the conventional monolayer resist method is employed. In such semiconductor elements there are irregularities on the substrate surface on which the pattern is to be made. Forming an accurate pattern of submicron level on such a substrate surface is very difficult on account of the variation of development condition resulting from the difference of resist film thickness, and the irregular reflection and standing wave of the exposing light that occur at the stepped part on the surface. In order to overcome the difficulty encountered in forming a submicron pattern on the stepped substrate, there has been proposed the multi-layer resist method. This method includes the three-layer resist method and the two-layer resist method In the case of three-layer resist method, the stepped substrate is coated with an organic flattening film, and then an inorganic intermediate layer and resist are placed on top of the other. A desired pattern is formed on the resist. Using the resist as a mask, the intermediate layer is subjected to dry etching. And finally, using the inorganic intermediate layer as a mask, a pattern is formed on the organic flattening film by O.sub.2 RIE. This method has long been studied because basically it can be carried out using the existing technology. However, it has a shortcoming of being very complex in process. Another disadvantage of this method is that cracks and pinholes are liable to occur in the intermediate layer because the organic film, inorganic film, and organic film of different properties are placed on top of the other. In contrast to the three-layer resist method, the two-layer resist method does not suffer cracks and pinholes because it employs a resist having the properties of both the resist and inorganic intermediate layer used in the three-layer resist method. In other words, this resist is resistant to oxygen plasma. The two-layer method is simpler in process than the three-layer method. Unfortunately, the conventional resist (top layer resist) used in the three-layer resist cannot be used in the two-layer resist method. Thus it is necessary to develop a new resist having resistance to oxygen plasma.
For reasons mentioned above, there has been a demand for a positive type photoresist having good resistance to oxygen plasma, high sensitivity, and high resolution which can be used for the top layer resist in the two-layer resist method and also can be developed with an alkali without the change of the existing process.